Foundry sand compositions containing graft copolymers of acrylic acid with methyl cellulose

ABSTRACT

A SAND CORE AND MOLD COMPOSITION FOR USE IN CASTING METALS COMPRISING FOUNDRY SAND, CLAY, WATER AND FROM ABOUT 0.0025% TO ABOUT 1% BY WEIGHT OF A WATERSOLUBLE GRAFT COPOLYMER OF ACRYLIC ACID AND A MINOR AMOUNT OF METHYL CELLULOSE, INCLUDING WATER-SOLUBLE SALTS THEREOF.

United States Patent O 3,577 367 FOUNDRY SAND COMPOSITIONS CONTAININGGRAFT COPOLYMERS F ACRYLIC ACID WITH METHYL CELLULOSE William J. Lang,Libertyville, Ill., assignor to International Minerals & ChemicalCorporation No Drawing. Filed Sept. 4, 1968, Ser. No. 757,469 Int. Cl.'C08f 29/3 6; BZZc 1/26 U.S. Cl. 260-17 15 Claims ABSTRACT OF THEDISCLOSURE A sand core and mold composition for use in casting metalscomprising foundry sand, clay, water and from about 0.0025% to about 1%by weight of a watersoluble graft copolymer of acrylic acid and a minoramount of methyl cellulose, including water-soluble salts thereof.

BACKGROUND OF THE INVENTION This invention relates to foundry sandcompositions for preparing mold and cores. More particularly, thisinvention relates to foundry sand compositions which have betterworkability characteristics and produce fewer defective moldings thanfoundry sand compositions of the prior art. Still more particularly,this invention relates to the use of water-soluble graftcopolymerization products of acrylic acid and a minor amount of methylcellulose, and water-soluble salts thereof, to impart superiorworkability characteristics to foundry sand compositions.

Conventional foundry molding sands are prepared by adding a bondingclay, such as a bentonite or fire clay or a combination of such clays,and a critically controlled amount of water to a classified sand whichmay be of a washed, crude or reclaimed variety. It is also commonpractice to employ an organic binder such as cereal or dextrine, and acombustible material such as sea coal. Deficiencies in the molding andworking properties of the foregoing general type of molding compositionshave presented a constant challenge to the foundry industry.

The foundry industry, therefore, has attempted numerous variations infoundry sand compositions with respect to the binder employed, but ithas been a general result that the different binders utilized haveimproved certain molding characteristics while degrading others. Thedifferent binders utilized in foundry sand compositions have includedsynthetic water-soluble polymers. The polymers of the prior art whichhave been used as binders in foundry sand mold and core compositionshave been described as improving properties of the compositions, e .g.,as giving good bakeability, good permeability to gases generated duringthe baking operation, good blendability or the ability to mix well anddeposit well upon blowing, and the like. Many of the polymers of theprior art having the above-mentioned properties are deficient in one ormore of other essential properties, namely, good green strength, goodhot strength, and good deformation characteristics. It was, therefore,impossible to select a single polymer which would produce a good balanceof the essential properties of a foundry sand composition.

As illustrative of the types of polymers suggested for use in foundrysand compositions, U.S. Pat. No. 2,817,128 teaches the use of awater-soluble polyelectrolyte, as for example, a polyacrylic acid,instead of organic binders such as cereal and dextrine. Thesepolyelectrolytes are described as improving the workability andflowability of the foundry sand in addition to making the water contentof such compositions a less critical variable. An improvement over theuse of these polyelectrolytes is described 3,577,367 Patented May 4,1971 in U.S. Pat. No. 3,104,230 which teaches the use of certaininorganic compounds in combination with the polyelectrolytes to impartsuperior hot strength properties to the sand compositions. Theseinorganic compounds include phosphoric acid the various alkali metal andalkaline earth metal salts of the oxy-acids of phosphorous and theoxy-acids of sulfur, such as tetrasodium pyrophosphate.

Although the aforementioned inorganic compounds provide improvementsover the sole use of the polyelectrolytes as binders, the foundry sandcompositions of U.S. Pat. No. 3,104,230 still suffer from certaindeficiencies. For example, while the use of the aforementioned inorganiccompounds improved the hot strength of the foundry sand compositions,the hot strength is not improved to the extent desirable. Theutilization of the invention of U.S. Pat. No. 3,104,230 suffers from thefurther disadvantage that the patent teaches it is necessary that amajor proportion of the total clay in the composition be of the Westernbentonite type. This limitation presents an economic disadvantage inareas Where the sub-bentonitic type of clay is more readily available.

As illustrative of another type of binder suggested for use in foundrysand compositions, U.S. Pat. No. 3,179,523 teaches the use of amagnesium or Zinc oxysalt cement in combination with an organic bindersuch as a watersoluble polysaccharide or water-soluble polysaccharidederivative.

SUMMARY OF THE INVENTION This invention is based on the discovery thatwatersoluble graft copolymerization products of acrylic acid and a minoramount of a methyl cellulose substrate, and water-soluble salts thereof,will impart superior workability characteristics to foundry sandcompositions. The foundry sand core and mold compositions of thisinvention comprise a major amount of foundry sand and minor amounts ofclay, water, and a graft copolymer of this invention, the graftcopolymer being present in an amount sufficient to enhance theworkability characteristics of the composition. More specifically, thesand compositions of this invention consist essentially of from about 84to about 96% by weight of a foundry sand, from about 3 to about 10% byweight of clay, from about 1 to about 5.5% by weight of water, and fromabout 0.0025 to about 1% by weight of a suitable graft copolymer.

The incorporation of the graft copolymer in the foundry sand compositionis effective to impart a good balance of essential molding properties.The graft copolymer achieves the balance of desirable properties withoutthe deleterious,

effects which commonly result from the use of polymers of the prior art.The foundry molding sand compositions.

formulated in accordance with the present invention exhibit good greenstrength, deformation characteristics and hot strength. In addition, thecastings produced by utilizing cores and molds made from thecompositions of this invention exhibit fewer scabs and other surfacedefects than when sand compositions of the prior art are employed.

It is, therefore, a primary object of this invention to provide animproved foundry sand composition.

Another object of this invention is to provide a foundry sandcomposition having superior workability characteristics.

Still another object of this invention is to provide a foundry sandcomposition with superior hot strength properties.

A further object of this invention is to provide a method for preparingfoundry sand compositions which includes incorporating in thecompositions a minor but effective amount of a water-soluble graftcopolymer of acrylic acid and a minor amount of methyl cellulose,including watersoluble salts thereof.

These and further objects of this invention will be described or beapparent as the description thereof herein proceeds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The graft copolymers which areuseful in the method of this invention are water-soluble graftcopolymers prepared by copolymerizing acrylic acid with a minor amountof methyl cellulose, and water-soluble salts thereof. The amount of themethyl cellulose which is reacted with acrylic acid may vary betweenabout 0.1 and about 10.0% preferably between about 0.25 and about 5.0%,based on the total weight of the acrylic acid used.

The graft copolymerization can be carried out by using well-knownpolymerization techniques. Ultraviolet light or any of the well-knownperoxygen-type initiators, e.g., peroxide free radical initiators, maybe used. The preferred peroxygen-type initiators are hydrogen peroxideand hydroperoxides such as t-butyl hydroperoxide, diisopropylbenzenehydroperoxide, cumene hydroperoxide, I- phenylethyl hydroperoxide, andthe like. Other useful peroxide initiators are diacyl peroxides such asbenzoyl peroxide and acetyl peroxide, and dialkyl peroxides such asdi-t-butyl peroxide and dicumyl peroxide. Still other usefulperoxygen-type initiators include per-salts such as sodium, potassium orammonium persulfate and sodium perborate; the peresters such as t-butylperoxyacetate and t butyl peroxybenzoate; and the peracids such asperformic acid, peracetic acid, perbenzoic acid, and peroxylactic acid.If desired, Redox acitvated systems can be used in accordance with theusual polymerization practices. Thus, sodium bisulfite-potassiumpersulfate and hydrogen peroxide-ferrous ion systems may be employed.However, the incremental addition of the peroxygen-type initiator ispreferred when a Redox activated system is utilized.

The quantity of the initiator employed can be varied depending on thereaction temperature and other conditions, but will ordinarily be fromabout 0.0005 to about 0.01%, preferably from about 0.002 to about0.004%, based on the weight of the acrylic acid.

The temperature of the reaction is not critical and may vary betweenabout --5 C. and about 100 C. The preferred temperature range is betweenabout 40 C. and about 80 C., with a temperature 'between about 60 C. andabout 70 C. being most preferred. The reaction may be carried out undersuperatmospheric pressure or even under partial vacuum. However, it ispreferred to utilize atmospheric pressure for convenience since thereaction runs very favorably at this pressure.

The graft copolymerization reaction is carried out in an acidic aqueousmedium. The pH of the reaction medium may be any value up to andincluding about 3.5. It is preferred to maintain the pH between about3.0 and about 3.5 for optimum results. In the event the amount ofacrylic acid utilized is not sufficient to lower the pH of the reactionmedium to a value of 3.5 or lower, the desired pH may be obtained by theaddition of a suitable mineral acid such as sulfuric acid, nitric acidor hydrochloric acid.

The reactants and the initiator are combined in the aqueous reactionmedium in any conventional manner. However, the initiator should not beadded to the reaction medium containing only the acrylic acid since thiscould cause the polymerization of the acrylic acid in the absence of themethyl cellulose. As illustrative of a suitable manner of combining thereactants, the acrylic acid and methyl cellulose are each dissolved inseparate amounts of water so as to provide homogeneous solutionsthereof. These solutions are mixed and the initiator is then added tothe mixed solution. Alternatively, the methyl cellulose may 'bedissolved in water to provide a homogeneous solution to which theacrylic acid and the initiator are added. Other methods of combining thereactants and initiator will be obvious to one skilled in the art.Optimum results will be obtained if the methyl cellulose is thoroughlydissolved in at least a part of the aqueous reaction medium before it iscombined with the acrylic acid. Stirring or shaking of the reactionmixture Will facilitate the polymerization and result in more uniformpolymers.

The reaction time will vary according to the reaction temperature and/orquantity of the initiator present. In general the time should besufiicient to consume at least of the acrylic acid. To illustrate thevariance of the reaction time at different temperatures, polymerizationwill be evident in about 5 to 7 hours when acrylic acid is reacted at 60C. with about 1.0% by weight of the methyl cellulose, based on theweight of the acrylic acid, and 0.002% by weight of hydrogen peroxide,based on the weight of acrylic acid, is present. On the other hand,polymerization will be complete in about 15 to 20 minutes when thereaction is carried out at C. using the same relative amounts of thereactants and the initiator. Polymerization may be detected by a changein refractive index, and completion of the reaction may be detected bythe absence of the distinctive odor of acrylic acid.

At the completion of the polymerization reaction, the copolymer thusproduced may be converted to the salt form. This conversion is generallyeffected by neutralization with an appropriate hydroxide. Ammonium oralkali metal salts of the copolymers such as sodium, potassium andlithium salts may be obtained by reacting the copolymers with thecorresponding hydroxide. Where complete neutralization is desired, astoichiometric amount, and preferably a slight excess, of the hydroxideis used. Partial salts may, of course, be produced by using less thanthe quantity of alkali metal or ammonium hydroxide to effect completeconversion of the carboxyl groups.

The copolymers, in either the acid form or the salt form, may beconverted to a dry form if so desired. This may be accomplished byconventional techniques, such as by heating the aqueous solution of thecopolymer in a drum drier at a temperature of about C. to drive off thewater and yield a dry polymer.

The graft copolymers may be used in accordance with this invention ineither the acid form or the salt form (e.g., as ammonium and alkalimetal salts). The amount of the graft copolymer to be used may be variedconsiderably. As little as about 0.0025 by weight of the green foundrysand composition, produces satisfactory core or mold mixes for somepurposes, while in others amounts as large as about 1% or more may beutilized. In general, from about 0.01 to about 0.5% by weight will befound to be satisfactory for most purposes, with from about 0.05 toabout 0.1% by weight being particularly preferred.

The foundry sand compositions of this invention will generally containfrom about 3 to about 10% by weight of clay, but lesser and greateramounts of clay will be used in certain applications. However, theamount of the clay in most instances will range from about 5 to about7.5% by weight. Standard foundry refractory and bentonite-type clays maybe utilized in the compositions of this invention, but it is preferredthat a major proportion of the total clay be a bentonite type of clay.Western or natural sodium bentonitic and native or sub-bentonitic claysare useful. The Western clays impart better workability char acteristicsto the core and mold compositions than the subbentonitic clays. Thedifference in the properties imparted to the sand compositions isapparently due to differences in the chemical compositions of the clays.Sub-bentonitic clays are generally calcium or magnesium varieties ofmontmorillonite and may contain substantial proportions of non-clay ornon-montmorillonite impurities. The Western bentonitic clays, which arenatural sodium clays,

are sometimes herein referred to merely as bentonitic clays.

When a sub-bentonitic or calcium montmorillonite type of clay is used,the clay is preferably first treated with an alkali metal compound whichis water-soluble, ionizable, and has an ion capable of reacting withcalcium to form a water-soluble precipitate. Such compounds include, asfor example, alkali metal carbonates, hypophosphates, oxalates, borates,phosphates, silicates, sulfites and tartrates. Sodium carbonate or sodaash is particularly preferred. The amount of the alkali metal compoundemployed will be somewhat dependent upon the proportion of calciummontmorillonite in the clay. In general, amounts of from about 0.5 toabout 6% by weight of the clay will be used. The preferred quantitiesfor treatment are from about 2 to about 4% by Weight of the clay.

The sand employed can be any of the standard foundry sands of thewashed, crude or reclaimed varieties. It is generally preferred to use aclean, washed roundor subangular grained silica base sand. Such sand isavailable in a wide range of particle sizes containing particles rangingfrom about 30 to about 270 mesh or finer U.S. series. The sand will bepresent in the composition in the amount of from about 84 to about 96%by weight of the composition, exclusive of combustible materials such assea coal, with a range of from about 89 to about 93% being preferred.

The only other ingredient required in the sand mix of this invention inaddition to the graft copolymer, clay and sand is water. In general,moisture is required to impart moldability to the mix and for thedevelopment of satisfactory green strength. Satisfactory green strength,moldability, and surface finish are obtained when there is from about 1to about 5.5% by weight total moisture in the mix. A total moisturecontent in the range of from about 2 to about 4% by weight isparticularly preferred.

The compositions of this invention may contain ingredients in additionto the aforementioned four essential ingredients, i.e., graft copolymer,clay, sand and water in the amounts indicated. Other ingredientsconventionally used in the foundry practice may be present in minoramounts, as for example, thermoplastic and thermosetting natural andsynthetic resins, flour or cereals, zirconite, sea coal and the like.For example, the sand compositions hereinbefore defined may be mixedwith a combustible material such as sea coal in an amount of up to about14% by weight, or even higher, of the total weight of the four essentialingredients. If desired, a minor amount, e.g., from about 0.05 to about1% by weight of the composition, of a water-soluble borate such as asodium borate may be used to further increase the green strength andimprove response to mulling. In addition, the foundry mold and core sandcompositions may also contain a minor amount, e.g., from about 0.05 toabout 0.5% by weight of the composition, of one of the inorganiccompounds described in US. Pat. No. 3,104,230 as being useful incombination with polyelectrolytes. Therefore, it will be understood thatthe term consisting essentially of when used herein is intended toindicate the named ingredients of a composition plus other materials ofa nature and in such amounts as will not substantially adversely aifectthe properties of the composition for its intended purposes, such asdescribed above.

In the practice of this invention, the ingredients of the sandcomposition may be mixed in any conventional manner and in any order.However, as hereinbefore pointed out, it is preferred that any alkalimetal compound such as soda ash be added to the clay before the clay iscombined with any of the other ingredients of the composition when asub-bentonitic clay is used. The graft copolymer in a dry finely dividedform may be mixed with dry sand and clay to form a composition to whichonly the correct amount of water need be added before use. Inasmuch asthe graft copolymers are water-soluble in both the acid and salt forms,they may be added in an aqueous solution to a dry mixture of the sandand clay. In still another alternative method of mixing the ingredients,the acid or salt form of the graft copolymer may be added in the dryform or in an aqueous solution to the clay before the clay is combinedwith the sand. When an aqueous solution of the graft copolymer is addedto the sand and/or clay, the concentration of the solution may beadjusted to yield a core or mold mix of the desired total moisturecontent.

Intermixture of the ingredients of the composition of this invention maybe performed by hand, in an internal mixer, in a paddle type mixer, orin any of the low and high speed mulling machines conventionallyemployed for preparing such compositions. No particular care as to timeand temperature of mixing need be exercised. An efficient internal mixeror mulling machine produces a homogeneous composition in a very shorttime, usually from about one-half to about four minutes.

Once a uniform and homogeneous molding composition has been prepared bythorough mulling and mixing of the aforementioned ingredients, it isready for use. It is poured or forced under pressure into a suitablecore or mold boX, or on a pattern or other shaping device, and rammed ortamped in place sufficiently to completely fill all recesses. Whencompacted or shaped in such a manner the core or mold is easily removedfrom the shaping device and is found to be hard and of such high greenstrength that it is easily handled in subsequent operations.

The green core or mold, which may be dried or baked, if necessary, isready for use. It may be dried at room temperature, or it may be bakedin an oven or dried with a torch if the air drying is undesirably slow.The operation in which the green core or mold is converted to a hard,dry condition is essentially a dehydration operation.

The following non-limiting example will serve to further illustrate thisinvention.

In the example the following terms are defined in accordance With thestandards of the American Foundrymens Society (hereinafter referred toas A.F.S.), and they represent various quality criteria of foundrymolding compositions as determined by standard test methods andspecifications of the A.F.S. The term moldability index is an indicationof the state of workability of the sand in molding for metal castings.This is determined by utilization of a Harry W. Dietert Companymoldability tester No. 875. The term green hardness is defined as theresistance offered by the surface of a green mold to deformation in asmall area. This measurement was made with a Harry W. Dietert Companygreen hardness tester No. 473. The term permeability as used hereinrefers to the physical property which determines the ability of gases toflow through molded masses of foundry molding compositions. Thepermeability test was made with 21 Harry W. Dietert Company permmeteraccording to the A.F.S. standard permeability procedure. The term greencompression strength is defined as the pressure in pounds per squareinch required to cause collapse of a standard rammed cylinder of undriedsand composition. The term deformation is the change in linear dimensionof a foundry molding composition test specimen when it is subjected toan applied stress. This measurement was taken with a Universal sandstrength machine No. 405. The term dry compression strength is definedas the pressure in pounds per square inch required to cause collapse ofa standard rammed cylinder of the sand composition after being dried.The term green shear A first foundry molding sand composition wasprepared in accordance with this invention by thoroughly mulling 92.3parts by weight of A.F.S. No. 56 sand, parts by weight of a Westernbentonite, 2.12 parts by weight of Water, and 0.1 part by weight of asodium salt of an acrylic acid-methyl cellulose graft copolymer preparedas outlined above. A second sand composition was prepared by followingthe teachings of the prior art for the purpose of comparison with thecomposition of this invention. This second composition was prepared bythoroughly mulling 98.6 parts by weight of A.F.S. No. 56 sand, 2.22parts by weight of Water, and 5 parts by weight of a mixture comprising100 parts by weight of a Western bentonite, 0.1 part by weight of asodium polyacrylamide and 0.1 part by weight of tetrasodiumpyrophosphate. Portions of each of the resulting compositions were thentested for moldability, green hardness, permeability, green compressionstrength, deformation, dry compression strength green shear strength andhot strength in accordance with the standards of the A.F.S. The resultsof these test are summarized in the following table.

It is apparent from these results that substantial improvements in theworkability of foundry sand compositions may be achieved byincorporation therein of a minor amount of a graft copolymer of thisinvention. For example, it will be noted that the composition of thisinvention exhibited superior moldability, permeability, greencompression strength, deformation characteristics, dry compressionstrength and hot strength as compared to the composition of the priorart.

Although this invention has been described in relation to specificembodiments it will be apparent that obvious modifications may be madeby one skilled in the art without departing from the intended scope ofthe invention as defined by the appended claims.

What is claimed is:

1. A foundry molding composition comprising a major amount of foundrysand and minor amounts of clay, water, and a water-soluble graftcopolymer, said graft copolymer being present in an amount sufiicient toenhance the workability of said composition and being of the groupconsisting of a graft copolymerization product of acrylic acid and fromabout 0.1 to about by weight, based on the weight of said acrylic acid,of methyl cellulose, and water-soluble salts of said graftcopolymerization product.

2. A foundry molding composition in accordance with claim 1 containingfrom about 0.0025 to about 1% by weight of said graft copolymer, andsaid graft copolymerization product is an ultraviolet light orperoxygen-type compound induced graft copolymerization product.

3. A foundry molding composition in accordance with claim 2 comprisingfrom about 84 to about 96% by weight of said foundry sand, from about 3to about 10% by weight of said clay, and from about 1 to about 5.5% byweight of water.

4. A foundry molding composition in accordance with claim 3 wherein saidgraft copolymerization product is a peroxide free radical induced graftcopolymerization product.

5. A foundry molding composition in accordance with claim 4 wherein saidgraft copolymerization product is a hydrogen peroxide or hydroperoxideinduced graft copolymerization product of acrylic acid and from about0.25 to about 5% by weight, based on the weight of said acrylic acid, ofmethyl cellulose.

6. A foundry molding composition in accordance with claim 4 includingsaid graft copolymerization product.

7. A foundry molding composition in accordane with claim 4 including awater-soluble salt of said graft copolymerization product.

8. A foundry molding composition in accordance with claim 7 including analkali metal salt of said graft copolymerization product.

9. A foundry molding composition in accordance with claim 8 including asodium salt of said graft copolymerization product.

10. A foundry molding composition in accordance with claim 4 wherein amajor proportion of said clay is a bentonitic clay.

11. A foundry molding composition in accordance with claim 4 wherein amajor proportion of said clay is a subbentonitic clay treated with fromabout 0.5 to about 6% by weight, based on the weight of said clay, ofsoda ash.

12. A foundry molding composition in accordance with claim 5 comprisingfrom about 89 to about 93% by Weight of said sand, from about 5 to about7.5% by weight of said clay, from about 2 to about 4% by weight ofwater, from about 0.01 to about 0.5% by Weight of said graft copolymer,and from about 0.05 to about 1% by weight of a water-soluble borate.

13. A foundry molding composition in accordance with claim 5 comprisingfrom about 89 to about 93% by Weight of said sand, from about 5 to about7.5% by Weight of said clay, from about 2 to about 4% by weight ofwater, and from about 0.05 to about 0.1% by weight of said graftcopolymer, a major proportion of said clay being a sub-bentonitic claytreated with from about 0.5 to about 6% by weight, based on the weightof said subbentonitic clay, of soda ash.

14. A foundry molding composition in accordance with claim 5 comprisingfrom about 89 to about 93% by weight of said sand, from about 5 to about7.5% by weight of said clay having a bentonitic clay as a majorproportion thereof, from about 2 to about 4% by weight of water, andfrom about 0.05 to about 0.1% by weight of said graft copolymer.

15. A foundry molding composition in accordance with claim 5 comprisingfrom about 89 to about 93% by weight of said sand, from about 5 to about7.5 by weight of said clay having a bentonitic clay as a majorproportion thereof, from about 2 to about 4% by Weight of water, andfrom about 0.05 to about 0.1% by weight of an alkali metal or ammoniumsalt of said graft copolymerization product.

References Cited UNITED STATES PATENTS 2,817,128 12/1957 Wickett 164-432,999,056 9/ 1961 Tanner 204-154 3,179,523 4/1965 Moren 10638.353,457,198 7/1969 Sobolev 2602.2

WILLIAM H. SHORT, Primary Examiner L. M. PHYNES, Assistant Examiner US.Cl. X.R.

